This invention pertains to apparatus and techniques for detecting the position of an electron beam in a cathode ray tube (CRT) using a semiconductor photodetector, and to application of these techniques to automatic convergence in color CRT displays.
It is well known in the art of conventional three-gun, shadow-mask type, CRT displays, that the image produced will contain certain inherent distortions if dynamic corrections are not applied. These distortions include, for example, pincushion distortion caused by the center of deflection of the three electron beams being located apart from the center of curvature of the tube's display screen (present in mono-chromatic as well as color CRT's), trapezoidal distortion caused by at least two of the electron guns being located off the longitudinal axis of the tube envelope, and misconvergence of the beams at the tube's shadow-mask caused by the guns being displaced from one another laterally. With a delta-gun configuration, all three guns are spaced about the longitudinal axis of the gun assembly; with an in-line configuration, one gun is located on axis and the other two are spaced at either side.
The usual method of correcting geometric distortion is to impress certain analog correction factors onto the deflection signals used to deflect the beam or beams back and forth across the display screen to produce the image raster. Misconvergence is usually corrected by a similar impression of different analog correction factors onto the magnetic fields used to converge the three beams at the center of the screen. Of the two distortions, the most difficult to correct accurately and uniformly, and the one which requires periodic adjustment, is that of misconvergence.
Basic schemes for accomplishing dynamic beam convergence include the production of individual vertical and horizontal signals for each of the beams within the tube. Approximating somewhat the form of slightly skewed parabolas, the correction signals provide zero correction at the center of the screen and increasing correction as the beams are deflected away from center. Such a basic approach is usually adequate for a home television environment where viewers are not overly critical and viewing distances are on the order of 6 to 10 feet. In the field of information display, however, where viewers are more critical and viewing distances much shorter, and more importantly, where resolution requirements are much stricter, the amount of misconvergence left uncorrected by such a basic approach is unacceptable.
An improvement over the basic scheme described above is exemplified by the Model 4027 color graphics terminal produced by Tektronix, Inc., the assignee of the present invention, wherein the display screen is divided into several sub-areas and different correction signals, independently adjustable, are generated for each such division. Such an approach permits a more accurate convergence of the three beams over the entire area of the screen. In the Model 4027, the display screen is divided into nine sub-areas and the beams may be converged in each such area by the adjustment of three potentiometers, one for each beam. Although providing increased correction, such a scheme still requires the somewhat time-consuming adjustment of 27 different potentiometers; three for each of the nine sub-areas. Other known schemes divide the display screen into an even greater number of sub-areas (the Tektronix Model 690 color monitor, for example, uses thirteen) and requires the attendant adjustment of an even greater number of potentiometers. A common disadvantage of such schemes is the requirement for a human operator to assume full control of the display system for the time necessary to perform the several adjustments at each individual sub-area.
More recent developments include digital convergence schemes wherein correction information may be entered digitally, via a keyboard or other similar means, for conversion into analog signals producing the desired amount of beam adjustment. Examples of such schemes include those disclosed by Hallett et al., U.S. Pat. No. 4,203,051 and its companion, Sowter, U.S. Pat. No. 4,203,054, both of which are assigned to IBM, and the Model 382 color display developed by Systems Research Laboratories (SRL) of Dayton, Ohio. The IBM scheme is also described in an article by J. S. Beeteson, et al., "Digital System for Convergence of Three-Beam High-Resolution Color Data Display's" appearing at page 598 of the September 1980 issue of IBM J. Res. Develop., Vol. 24, No. 5. A description of the SRL convergence scheme may be found in a paper entitled "A 25-In. Precision Color Display for Simulator Visual Systems" by R. E. Holmes and J. A. Mays of SRL. A common characteristic of both the IBM and SRL systems is the use of a keyboard permitting operator entry of digital information representing the degree of movement necessary for each of the three beams to accomplish their convergence or other geometric adjustment. The IBM system permits the beams to be individually adjusted at 13 different points over the display area, while the SRL system permits adjustment at 256 different points.
A semi-automatic scheme for performing deflection adjustments only is disclosed by Bristow in U.S. Pat. No. 4,099,092. In that scheme, a photodiode array or solid-state imaging camera positioned in front of a CRT display, and a digital computer, are employed to generate correction factors for later application, via a programmable read-only memory, to the conventional deflection waveforms.
A common disadvantage of all the above schemes, however, is that a human operator is still required to assume full control of the system during the time necessary to perform the convergence or correction operation.
An expensive, but completely automatic convergence scheme is disclosed by Robinder, et al., in U.S. Pat. No. 4,456,853, assigned to Tektronix, Inc., which is especially useful for high resolution color graphic displays, such as the Tektronix Model 4115B Computer Color Display Terminal. In that apparatus, the tube is generally an otherwise conventional high resolution color CRT which includes a display screen of phosphorescent material, three electron guns for producing and directing electron beams toward the display screen, and a shadow mask. However, located on the back surface of the shadow mask, i.e., on the surface facing the electron guns, are a plurality of feedback elements constructed of phosphorescent material. In a preferred mode, these feedback elements are configured as two disjointed legs of a right triangle, one vertical and one inclined, spaced at preselected locations over the back surface of the shadow mask. As an electron beam is scanned across the face of the tube, the legs phosphoresce when struck by electrons in the beam, and the precise time of the phosphoresence of each leg is measured using a photomultiplier tube. The incremental time from the beginning of the raster to the time of phosphorescence of the vertical leg provides an indication of the horizontal position of the beam, while the incremental time from the beginning of the raster to the time of the phosphorescence of the inclined leg provides an indication of the vertical position of the beam. This information is obtained for each of the three electron beams and for each of the feedback elements, and the information is processed to provide convergence correction waveforms which are applied to the convergence magnet assembly and the deflection yoke (or plates) of the CRT.
Two important aspects of this latter technique are that the detector, a photomultiplier tube in the embodiment described above, have a very fast response in order to detect small changes in the incremental times involved as the raster traverses the legs, and that it be very sensitive in order to be able to detect the phosphorescence resulting from a single raster line on a single feedback element. These two requirements together result in a very expensive apparatus for autoconvergence and militates against the use of the more inexpensive detectors such as photodiodes, and other semiconductor-based photodetectors. As a general rule, such inexpensive devices can be made very sensitive, but at the expense of increased response time, or they can be made to have a very fast response time but with attendant degradation in sensitivity. At the present time, semiconductor-based photodetectors are not generally available which can achieve both the rapid response and the sensitivity required to simply replace the photomultiplier tube in the above application. What is needed is an inexpensive detection system for autoconvergence which does not require such high sensitivity or rapid response time.